Two teachers' unions said they have received legal advice that the Australian Curriculum, Assessment and Reporting Authority (ACARA) cannot require teaching staff to sign the documents and have concerns about the ramifications for teachers who may "inadvertently" breach the agreement.

However, ACARA and the NSW Department of Education said teachers involved in trialling NAPLAN test items had been asked to sign confidentiality agreements since 2008.

"This is to ensure the security of potential test content, so no student has any unfair advantage when taking NAPLAN the following May," a spokesman for ACARA, which runs national NAPLAN testing, said.

"The form primarily serves as a reminder of the professional and ethical obligation to keep potential test content confidential. It is not a legal document."

The NSW Teachers Federation and the Independent Education Union have advised their combined membership of more than 140,000 teachers and support staff not to sign an agreement stating that they will not copy or "disclose the content of the [NAPLAN online item-trial] test or any related materials and procedures to any other person(s) apart from Pearson and the students undertaking [the test]".

The unions, which represent teachers from government and non-government schools, said they also have concerns relating to the agreement's reference to Pearson, the private company which will administer the online NAPLAN tests.

Teachers Federation president Maurie Mulheron said he was not aware of teachers being asked to sign a similar agreement in past years.

"It's an individual, personal agreement signed with a third party, and the third party is not your employer, the NSW Department of Education," Mr Mulheron said. "There's no information on what would happen if you breached the agreement.

"We're concerned about the reference to Pearson, which are an aggressive company, based on experiences overseas. They have issued lawsuits against teachers and students."

Mr Mulheron said the union had received complaints about the agreement from teachers across NSW and that staff at a high school in Illawarra reported that they were asked by a Pearson invigilator to leave a briefing about the NAPLAN online test when they refused to sign the agreement.

Pearson did not respond to questions and referred the matter to ACARA, which did not answer questions about the role of the company.

Independent Education Union assistant secretary Mark Northam said he was also unaware of similar confidentiality agreements being issued in the past and that the document "can be construed as excessive".

"It's normal for a teacher who's involved in writing HSC tests to be asked to sign confidentiality agreements, but these teachers aren't generating [NAPLAN] test questions," Mr Northam said.

"For a standard teacher of an HSC subject, it is entirely good professional practice to sit around talking about an exam and what it means for their students.

"It would be very easy to inadvertently disclose something in a perfectly legitimate classroom discussion about a test.

"What this requirement seems to be doing is saying you best be very cautious."

NSW Secondary Principals' Council president and school principal Chris Presland also said he had never seen a similar agreement and that it was "inappropriate" for it to come from outside the NSW Department of Education.

"We have no direct relationship with ACARA and there's no discussion around what it's really intended to do," Mr Presland said.

The question trials are being conducted at 440 schools across the country this month, at the same time as all schools begin readiness tests to ensure they have the technical capacity to move to NAPLAN online. Students are also receiving their latest NAPLAN results this week.